Single-ended fluorescent lamps with two or more parallel tubes located adjacent each other are described in U.S. Pat. Nos. 4,481,442 and 4,375,607. A single base is located at the end portions of the respective legs. The cross or connecting portions of the discharge tubes are formed to provide for enhanced heat dissipation. In operation of the lamp, the ends of the discharge vessel remote from the electrodes form cool zones or cool spots, on which mercury of the fill of the lamps may condense.
The temperature of the coldest spot in the discharge vessel determines the mercury vapor pressure in operation of the lamp. The mercury vapor pressure is determinative of the light output from the lamp. Compact low-pressure discharge lamps as described in the referenced patents, when operated in vertical "base up" position, i.e. with the base being upwardly of the connecting corner forming the cool spots in the lamp, or in horizontal position, with the base being laterally of the cool spots, will result in an optimum temperature at the cooling spot of about 45.degree. C. The discharge within the lamp is non-varying and the light output is effectively constant or even.
If lamps of this type are operated vertically with the base being downwardly, that is, in "base down" position, the cooling zones in the cross portions of the lamp may reach a temperature which deviates from the optimum temperature of operation of the lamp. The temperature may rise. The light output for the lamp is thereby reduced. Vertical operation of the lamp, if the connecting portions or cool zones are substantially cooled by an ambient air current or air stream, however, may lead to formation of drops of mercury which condenses at the cooling spots or zones, resulting in variation or undulation of light output, and interference with smooth and uniform operation of the discharge therein. It is possible, in some conditions of position of the lamp, that condensing mercury drops directly on one or the other of the electrodes which, if such drops persist, may lead to damage of the electrodes. The dropping mercury, which is continuously vaporized by the discharge within the tube, that is, the mercury which continuously recondenses, may, under some operating conditions, even lead to flaking or removal of fluorescent material with which the inside of the discharge vessel is coated.
It has previously been proposed to provide uniform cooling to a fluorescent lamp - see U.S. Pat. No. 4,375,607-- by surrounding a single-ended fluorescent lamp with an outer bulb, spaced from the discharge vessel itself. The discharge vessel is located wholly outside of the base and is formed by a U-shaped tube which is bent three times, and includes a cooling tip, located just above an attachment plate which closes off the base with respect to the discharge vessel. Peripheral slits, and an opening beneath the cooling tip in the attachment plate, together with vent openings in the lower portion of the base and in the bulb in a region remote from the base, permit an air stream within the lamp, that is, air convection therein. The air flows past the cooling tip, and thereby leads to condensation of the mercury at this cooling tip.
The lamp just described requires the outside surrounding bulb to obtain condensation of the mercury in the cooling tip, since the flue effect or thermal duct effect can be obtained only by the outer bulb by insuring sufficient air flow required for condensation of the mercury at the cooling tip. The provision of the outer bulb, however, substantially increases the size of the discharge lamp, which then no longer will be highly compact, and thus cannot be used anymore in various fixtures which are designed specifically for low-pressure compact discharge lamps.